Integrase (IN) is one of three viral-encoded enzymes that are essential for retroviral replication and, therefore, an important target for the development of strategies for the treatment of AIDS. Although some HIV-1 IN inhibitors with antiviral activity have been identified by us and others, progress has been seriously limited by a lack of critical details concerning the molecular structure of the active IN complex and its interactions with viral and host DNA substrates. The new approaches described in this competing renewal are designed to close this gap, benefiting from experimental systems developed to study ASV IN. The first two Aims address specific features of IN-DNA interactions: Aim 1 will identify ASV and HIV-1 IN residues that promote or stabilize viral DNA end unpairing, a step required for processing, by using chemical probing, biophysical methods, and mutagenesis. Aim 2 will map specific contacts between IN and viral and target DNA using new photo crosslinking methods together with molecular models of IN-DNA complexes. An iterative process of modeling and testing will identify all relevant contact sites between IN and substrate DNAs. In Aim 3 ASV IN proteins and IN-DNA complexes will be analyzed by dynamic light scattering and small angle X-ray scattering methods to determine their composition and conformation in solution. Chemical trapping methods will be used to prepare covalently linked, biologically-relevant IN-DNA complexes for structural analysis. The results of these analyses will confirm the stoichiometry and spatial orientation of IN domains and substrate DNAs in these complexes and support the modeling process of Aim 2. Homogeneous complex preparations will be tested in crystallization trials with the goal of acquiring structural data at atomic resolution. Predictions concerning structure and mechanism derived from these studies will be tested with HIV-1 IN. Experiments in this proposal are designed to provide a detailed understanding of the molecular interactions between HIV-1 IN and its DNA substrates. Such information is crucial for exploiting this viral protein as a target for the design of new therapies against AIDS.